414 ELEMENTS OF BIOLOGY 



adult animal that consists solely of one organ system, namely, the 

 reproductive, derived from a free-swimming larva not greatly dif- 

 ferent from that of other arthropods of its type, can be explained 

 only on the assumption that as the animal acquired the parasitic 

 habit, degeneration of adult structures, other than the reproductive 

 system, occurred. In other words, the animal as it exists today is a 

 product of descent with change, the changes in this case beinj 

 degenerative. 



The presence in parasitic organisms of vestiges of structures that 

 are functional in related free-living forms is to be regarded as an 

 expression of the history of the parasites, not different in principle 

 from the significance of the vestigial structures found in animals 

 in general. 



Evidence from the Distribution of Animals. The dis- 

 tribution of animals is dependent on several conditions: 



(i) Barriers: Long distances, wide bodies of water, mountainous 

 regions, and sharp differences in climate serve to restrict the wan- 

 derings of animals, the degree of restriction depending more or less 

 upon the effectiveness of locomotion of the animals. None of these 

 except climate are absolute restrictions on the migrations of birds, 

 because of their powers of flight. On the other hand, the mammals 

 are slow to distribute themselves over land areas and are completely 

 bound by wide water barriers because of their comparatively slow 

 and ineffective method of locomotion. 



(2) Thne: The older the form, that is, the earlier its appearance 

 among geological records, the wider its distribution. For example, 

 the paddle fish, Polyedon spatula is found only in the Missis- 

 sippi River and its tributaries. It belongs to one of the lower 

 and more primitive groups of fishes. Extinct members of the group 

 are found rather widely distributed, but its nearest living relative 

 is a fish found only in the Yangtse River in China, the two forms 

 being separated by half the circumference of the earth. The family 

 Hypochilidx of the spiders is represented by three living species; 



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